1. Field of the Invention
The present invention relates to feedwater spargers in boiling water reactors and, more particularly, to clamps for the end bracket assemblies of feedwater spargers and to methods of preventing separation of feedwater sparger end bracket assemblies.
2. Brief Discussion of the Related Art
Conventional boiling water reactors typically include a reactor vessel, a shroud disposed within the reactor vessel and a fuel assembly within the shroud. Feedwater enters the reactor vessel via a feedwater inlet or nozzle and is distributed circumferentially within the reactor vessel by a feedwater sparger disposed in the reactor vessel between the shroud and the reactor vessel wall. The feedwater sparger comprises a ring-shaped pipe or conduit for carrying the feedwater and having an end attached to a sparger end plate via a feedwater sparger end weld, the sparger end plate and conduit end attached thereto defining an end of the feedwater sparger. A feedwater sparger end bracket assembly couples the end of the feedwater sparger to the reactor vessel wall in spaced relation therewith. The feedwater sparger end bracket assembly normally comprises an attachment plate connected to the sparger end plate via a weld, and the structural components of the feedwater sparger end bracket assembly are ordinarily connected to one another via one or more additional welds. The attachment plate and the sparger end plate attached thereto form a sparger/bracket junction by which the conduit of the feedwater sparger is connected to the feedwater sparger end bracket assembly. The feedwater sparger end bracket assembly defines a load path for transferring loads from the feedwater sparger to a reactor vessel attachment fitting attached to the reactor vessel wall and to which the feedwater sparger end bracket assembly is connected. The structural adequacy of feedwater sparger end welds and feedwater sparger end bracket assembly welds has been questioned in light of cracking identified in these welds. In particular, the weld between the attachment plate and the sparger end plate and the weld between the sparger end plate and the conduit end are primarily fillet welds, and reactor coolant can infiltrate or get between the structural components joined by these fillet welds so that the roots of the fillet welds are exposed to reactor coolant. The geometry of the fillet welds presents a crevice where corrosive products can concentrate and accumulate over time, thereby producing stress corrosion cracking.
Cracks large enough to allow significant flow of feedwater from the feedwater sparger may result in direct impingement of the relatively colder feedwater on the reactor vessel wall, causing thermal shock and cracking of the cladding on the interior surface of the reactor vessel wall. In addition, the feedwater sparger end bracket assemblies usually carry an installation preload, and this preload is undesirably compromised or lost in the event of cracking of the feedwater sparger end welds and/or the feedwater sparger end bracket assembly welds, especially in the event of cracking which results in complete detachment of the feedwater sparger end bracket assembly from the feedwater sparger. An example of feedwater sparger end bracket assemblies that have an installation preload are those associated with feedwater spargers that are sprung into place during installation, such as to maintain contact between flow baffles of the feedwater spargers and the reactor vessel wall. The flow baffles, which are ordinarily located at the feedwater nozzles in the reactor vessel, must remain essentially in contact with the reactor vessel wall to effectively eliminate thermal shock conditions at the feedwater nozzles. To assure this, the feedwater spargers are sprung into place at installation, resulting in an installation preload on each feedwater sparger end bracket assembly of about eight thousand pounds. In the event of complete weld failure causing the feedwater spargers to become completely detached from the feedwater sparger end bracket assemblies, the installation preload is lost and the feedwater spargers will not perform as designed.
Mechanical solutions to the problems of cracked feedwater sparger end welds and feedwater sparger end bracket assembly welds encounter numerous obstacles in that mechanical devices attached to the ends of the feedwater spargers and the feedwater sparger end bracket assemblies must be capable of maintaining the installation preload in the event of weld failure. Mechanical devices must be capable of balancing all loads and moments to which they are subjected, and particularly must react to the loads and moments created when there is a complete through wall crack of the feedwater sparger end welds and/or the welds of the feedwater sparger end bracket assemblies. Another deterrent to the use of mechanical devices to address the problems of weld failure in the ends of feedwater spargers and in feedwater sparger end bracket assemblies is that existing feedwater sparger end bracket assemblies often have different structural dimensions and/or components. The use of mechanical devices with feedwater sparger end bracket assemblies is thusly impeded by the difficulty involved in designing an essentially standard mechanical device for use with different feedwater sparger end bracket assemblies. A further impediment to the use of mechanical devices in response to cracking of feedwater sparger end welds and feedwater sparger end bracket assembly welds is the need for the mechanical devices to be installed using equipment or tooling operated from a location remote from the reactor vessel.